I played around with my Trans on the 77 300D a bit. You have a "TV" Rod no Vac-Mod. The TV rod connects to a turn buckle on the right side of the engine along with the "throttle" (Accellerator pedal) linkage. If you shorten the length of it will allow the trans to build less presure and shift earlier. If you lengthen it, it will shift latter but may slip or flare. You need to find the right combination. There is also a bolt that can be removed from the right rear of the trans that allows access to a sort of K1 type spring. You can tighten it to reduce slipping. But I heard that if it is to tight it can lock up trans, I haven't adjusted my spring yet. I'll search for a post I had that contained some very usful info on these Trannys, and post it for you.